1. Field of the Invention
The present invention generally relates to radiation image reading apparatuses and, more particularly, to a radiation image reading apparatus which stores radiation image information in a photostimulated phosphor and reads the image information by photostimulated luminescence light.
2. State of the Prior Art
Recently, radiation images such as x-ray images are widely used for making disease diagnosis. Conventionally, the so-called radiation photography is used as a means of obtaining the x-ray image. In other words, the x-ray transmitted through an object (for example, a patient) is irradiated on a phosphor layer or screen so as to generate visible light, and this visible light is irradiated on a silver film and developed.
On the other hand, as an x-ray image pickup system which can realize high sensitivity and high resolution, a technique which uses a photostimulated phosphor has been proposed to replace the conventional technique of recording a 2-dimensional radiation image directly or indirectly on a film which is made up of a sheet which is coated with a silver (silver chloride) sensitive agent.
The basic method of using the photostimulated phosphor is proposed in a U.S. Pat. No. 3,859,523. According to this proposed method, the photostimulated phosphor is made of BaBr.sub.2 :Eu, for example. When the photostimulated phosphor receives a radiation energy of the x-ray or the like, this energy is stored in the crystals of the photostimulated phosphor in a relatively stable storage state. The energy is stored in the photostimulated phosphor for a long or relatively long time.
When a first light, which acts as an excitation light, is irradiated on this photostimulated phosphor in this state Where the energy is stored therein, photostimulated luminescence light having an intensity corresponding to the stored energy is emitted from the phosphor as a second light.
In this case, the first light is not limited to visible light, and light having a wide range of wavelengths from the infrared ray to the ultraviolet ray, may be selected and used. This selection differs depending on the phosphor material. The second light may also range from the infrared ray to the ultraviolet ray, and the particular kind of second light depends on the phosphor material.
On the other hand, an x-ray image pickup system has been proposed that utilizes the characteristics of the photostimulated phosphor. According to the x-ray image pickup system, the radiation transmitted through the object, such as the patient, is irradiated on the photostimulated phosphor and recorded thereon, so that radiation image information is obtained.
More particularly, excitation light such as a laser beam scans the photostimulated phosphor plate (or sheet) which stores the x-ray information related to the object, so as to generate photostimulated luminescence light. This photostimulated luminescence light is converged and converted in a photoelectric converter, to obtain an electrical signal proportional to the intensity of the stored radiation. Thereafter, an image processing is carried out with respect to this electrical signal. In other words, a visualized radiation image is obtained by printing the image on a silver film or displaying the image on a cathode ray tube (CRT).
After the photostimulated luminescence light is generated by using the excitation laser beam, radiation energy still remains within the photostimulated phosphor plate. Hence, strong excitation light is irradiated on the photostimulated phosphor plate to eliminate the residual energy by generating photostimulated luminescence light. Then, the photostimulated phosphor plate again stores the x-ray information related to the object. Hence, the recording of the x-ray information, up to the erasure of the residual energy, corresponds to one processing cycle of the read apparatus, and such a processing cycle is repeated.